<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <input type="file" id="file">

  <script>
    const fileDom = document.querySelector("#file");


    fileDom.onchange = function () {

      const file = this.files[0];

      const formdata = new FormData();

      // 根据接口文档的要求 key 。。。。
      formdata.append("abc", file);//     formdata 来包装文件



    }

  </script>
</body>

</html>